A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC/DC converters are frequently employed to advantage. DC/DC converters generally include an inverter, a transformer, a rectifier on a secondary side of the transformer and an output filter. The inverter generally includes switching devices (power switches), such as field effect transistors (FETs), that convert a DC input voltage to an AC voltage. The transformer transforms the AC voltage to another value, and the rectifier generates the desired DC voltage to the output filter. Conventionally, the rectifier comprises a plurality of rectifying diodes that conduct the load current only when forward-biased in response to the input waveform to the rectifier. Finally, the output filter eliminates large fluctuations in the output voltage to provide an essentially constant DC voltage at the output of the converter.
A conventional current-fed DC/DC converter is often used for off-line power factor correction and battery applications due to its voltage step-up capability and its continuous input current characteristics. This conventional current-fed converter typically employs two power switches coupled to equal and opposite polarity windings on the primary side of a transformer. A traditional form of power switch control operates the two power switches in a 180 degree interleaved fashion with a duty cycle equal to or greater than 50 percent. When both power switches are on, the input voltage is applied to an input inductor causing its current to increase. Then, when one of the power switches is turned off, the current in the input inductor continues to flow through the connected primary winding thereby transferring power to an output load.
This causes the conventional current-fed converter to transfer power only during a portion of a switching cycle, which becomes even smaller as the input voltage becomes smaller. This characteristic of the conventional current-fed converter causes the output current to be discontinuous and to pulsate strongly. A discontinuous and strongly pulsating output current greatly increases the difficulty in designing both the transformer and the output filter, typically forcing them to be physically larger. Additionally, overall converter efficiency is also lower due to the discontinuous operating characteristics.
Accordingly, what is needed in the art is a way to improve the operating characteristics, overall efficiency of a current-fed converter, while maintaining or reducing component size at the same time.